1. Field of the Invention
The present invention relates to the field of wireless communications. More specifically, it relates to a system for automatically sensing changes/variations in receive path gain and compensating for those changes/variations when detected.
2. Description of the Related Art
In order for a wireless communications service provider to claim certain geographic areas (e.g., cells) of coverage, that service provider must be able to guarantee certain minimum criteria of the received signal within that geographic area. That is, the service provider must be able to guarantee a minimum sustained level of signal strength within an entire cell of its service area.
One of the limitations placed upon the service provider is the fact that its radio receivers at the cell site equipment are capable of detecting only those signals that are at least 10 dB above the noise level (a largely fixed value) for a time division multiple access (TDMA) system and those signals at or below the noise level for a code division multiple access (CDMA) system.
Typically, as depicted in FIG. 1, the receiving equipment at a cell site consists of a receiving antenna 100 mounted atop a cell site tower 140 which feeds, via receiver cable 130, into the cell site equipment 135. The cable length varies of course with the height of the cell site tower, however, typically this height can be between 20-200 feet. The cell site equipment 135 typically includes a receiver filter 105, a fixed gain low noise amplifier (LNA) 110, and the remainder of the receiver path 120.
The receiver filter 105, as is known in the art, is used for eliminating both signal noise and any of the signal that is not within a predetermined bandwidth. The output of the receiver filter 105 is then coupled to a fixed gain low noise amplifier (LNA) 110 for amplifying the received signal. The output of the LNA 110 is then coupled to the remainder of the receive path 120 of the cell site equipment 135 (e.g., radio frequency (RF) splitters, etc) and eventually to individual radio receiver ports, as is known in the art.
A problem with using the FIG. 1 receive path to feed into the cell site equipment 135, is that the noise figure experienced between the receiver antenna 100 and the filter 105/LNA 110 combination (i.e., over the 20-200 foot receiver cable 130) is high. In addition, there are variable losses within the cable 130 itself. These cable losses can vary with ambient conditions including temperature, etc. The higher the losses experienced within the cable 130, the lower the signal strength at the cell site equipment 135. For example, approximately 2 dB can be lost in the cable 130 between the receiving antenna 100 and the filter 105/LNA 110 combination. These losses contribute to reducing a service provider""s coverage area inasmuch as the coverage area is defined by, among other things, how well the strength of a received signal is maintained from the receiver antenna to the receiver radios and how well that provider can keep the noise figure associated with the received signal to a minimum.
Turning to FIG. 2, an alternative system for receiving wireless signals at a cell site is depicted for reducing the noise figure of the received signal. Similarly to FIG. 1, the FIG. 2 receiver contains the same receiving components (i.e., antenna 100, receiver filter 105, and fixed gain LNA 110), however, these components have been placed on top of the receiver tower 140 along with the receiver antenna 100 at the cell site. Placing the receiving equipment (i.e., filter 105/fixed gain LNA 110) on top of the tower 140, sometimes referred to as the masthead, results in a reduction of the noise figure of the received signal but does not change the receive path gain. These improvements allow the service provider to expand the geographic area that defines the cell site, thereby expanding wireless coverage while effectively using the same receiving equipment.
For example, instead of being able to detect only those signals that are 10 dB above the noise level, with the addition of the fixed gain LNA 110 at the masthead, the antenna may now receive all signals of e.g., only 7 dB above the noise level and still be able to detect those signals at the remainder of the receive path 120. This improvement effectively increases the radius of coverage of the cell.
However, a problem associated with the FIG. 2 receiving system is that the remainder of the receive path 120 is still at the bottom of the tower 140 within the cell site equipment 135, while the receiving components (i.e., antenna 100, filter 105/LNA 110) are mounted atop the tower 140 with approximately 20-200 feet of receiver cable 130 separating them. As a result, receive path gain, while somewhat improved by placing the filter 105/LNA 110 combination atop the tower 140, varies as a function of the instability of e.g., the RF losses and impedance of the receiver cable 130. This instability in the receive path gain effectively reduces the amount of coverage area in a cell site since the required minimum levels of signal strength are not maintained. That is, while the service provider should realize the full 3 dB advantage by placing the LNA 110 on top of the tower, only approximately 1 dB improvement can be realized due to the losses in the cable. Thus, a system and method for automatically sensing cable losses and compensating for those losses as measured at the cell site equipment 135 is desirable.
The present invention provides a system and method for automatically sensing cable losses and adjusting for those losses as measured at the cell site equipment from a receiver antenna and filter/LNA combination that is located remotely from the cell site equipment. A circuit is provided for sensing a voltage at an exit port to the cell site equipment. If the sensed voltage is different from a predetermined reference voltage, thereby signifying the presence of cable losses, a control signal is delivered from an error amplifier to a variable gain LNA for varying its gain and thereby adjusting (up or down) the strength of the received signal so as to maintain a constant receive path gain. The circuit also provides a source of power for the masthead containing a portion of the receiver circuitry atop the tower.